In xerographic print engines, a tone reproduction curve (TRC) is important for controlling the image quality of the output. An image input to be copied or printed has a specific tone reproduction curve. The image output terminal outputting a desired image has an intrinsic tone reproduction curve. If the image output terminal is allowed to operate uncontrolled, the tone reproduction curve of the image output by the image output terminal will distort the rendition of the image. Thus, an image output terminal should be controlled to match its intrinsic tone reproduction curve to the tone reproduction curve of the image input. An intrinsic tone reproduction curve of an image output terminal may vary due to changes in such uncontrollable variables such as humidity or temperature and the age of the xerographic materials, that is often correlated with the numbers of prints made since the developer, the photoreceptor, etc. were new.
Solid developed mass per unit area (DMA) control is a critical part of TRC control. If the DMA is too low then the images will be too light and customers will be dissatisfied. On the other hand, if the DMA is too high, then other xerographic or image quality problems, such as poor transfer efficiency, fusing defects, or toner scatter on lines, etc., can occur. High DMA will also increase the total cost to owner. Maintaining a constant DMA or a low variation of DMA has always been a challenge in xerographic process controls design.
In addition, in copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to create a “test patch” of a predetermined desired density. The actual density of the printing material in the test patch can then be optically measured by a suitable sensor to sample along the TRC. In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive photoreceptor, on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent. These test patches are used to estimate the deposition of toner on paper in order to control the tone reproduction curve. Often it is not desirable to use customer media (paper, etc.) for control, and so the measurements are frequently obtained by directly measuring the developed patches on the photoreceptor.
To reduce noise, sensor to sensor variability, and other measurement errors, sensor readings are typically normalized. However, conventional normalizing techniques may not be able to adequately alleviate or reduce measurement errors.